Study On Anti-floating Structures Of Super Large Underground Hub Along The Lower Reaches Of The Yangtze River

This paper takes the tunnel engineering of the Nanjing Youth Olympic Games as the research object.Based on an in-depth analysis of the engineering geology and hydrogeological conditions of the project site.Combining the morphological characteristics of the underground structure of the traffic tunnel,the deep excavation depth of the foundation pit,and the fluctuation of the structural floor,the project area geological division.Perform engineering geological analysis of each subarea,and then determine the anti-floating water resistance of each subdivision under extreme precipitation conditions,and then discuss specific anti-floating measures for each subarea.This study not only provides a basis for the anti-floating design of the underground traffic tunnel in the tunnel engineering of the Nanjing Youth Olympic Games,but also provides design experience for the anti-floating design of ultra-deep and ultra-large-span underground projects in the future.This article has achieved the following recognition and research results.(1)Through the analysis of regional geological data and engineering geology and hydrogeological conditions in the project area.ldentify the type of groundwater in the field,mainly loose pores and loose rock pore confined water.For the phreatic layer of the project area,the elevation of the water level is mainly affected by the atmospheric precipitation in Nanjing and the water level in the Nanjing section of the Yangtze River.For the confined aquifer in this project area.As the confined aquifer in the project area is cut by the deep trough of the Yangtze River.As a result,the confined aquifer in the site area directly exposed to the side of the Yangtze River.Therefore,the confined aquifer in this project area forms a "skylight" in the part of the Yangtze River.This results in the Yangtze River being the main source of recharge for the confined aquifer in the site area,and its excretion is mainly discharged to the Yangtze River.(2)The excavation depth of the foundation pit reaches a maximum of 27m,and the maximum height difference of the underground floor slab reaches 15m.The project can be roughly divided into two major areas in terms of space configuration.The first part is the underground three-level interchange section,and the other part is the main tunnel section of Meizizhou.(3)Based on regional geological data and engineering geological conditions of the site,according to the differences in the shape of the different sections of the underground structure,the positional relationship between the floor and the aquifer,and the fluctuation of the floor of the underground structure.The anti-floating zoning of underground structures is mainly divided into four subdivisions,in which subfloors I and III of subterranean structures are located in the same stratum(pressure aquifer)and are therefore similar subdivisions.The bottom plates of II and IV also belong to the same stratum(dive layer),and therefore belong to the same type of subarea.Therefore,the representative subdivision I and subarea II are mainly analyzed.(4)The MODFlow software was used to simulate the changes in the water level of the confined aquifer in the site area supplied by the Yangtze River under extreme precipitation conditions.(5)Based on the engineering geology and hydrogeological conditions of the site area,the main anti-floating measures of the project are divided into the following two aspects in accordance with the characteristics of each sub-zone.On the one hand,the enclosing structure during construction is used as anti-floating measures during the operation of the project.On the other hand,anti-floating components are laid directly in the underground structure.The anti-floating components of this project are anti-uploading piles.Finally,the FLAC3D numerical simulation analysis of the anti-floating measures in each subarea of the field area mainly leads to the following conclusions:1.The anti-floating measures program of this project is feasible and effective;2.The anti-floating utilization of the building envelope,in the laminated wall When designing a construction,attention should be paid to the stress and strain concentration at both ends of the structural floor.(6)For subterranean structures with ultra-deep and large horizontal spans,the anti-floating design of the sub-districts in the field is not only feasible and effective,but also can save engineering budgets by guaranteeing that it has adequate anti-floatity.